Mineral Considerations for Beef Cows

Profit Tip: Mineral Considerations for Beef Cows

January 2015

Feed costs, labor costs, fuel costs and machinery costs have contributed to the increase in cow costs. Mineral cost, especially phosphorus costs have increased substantially this year. Mineral costs are not a major costs as compared to other costs such as harvested feed costs, but as producers strive to remain competitive in a climate where costs continue to go up, they will need to push the pencil on every management decision.


The one mineral that should always be supplied to cows free-choice is salt. Range cows will consume .05 to .1 lb/cow/day. At these rates, a cow will consume 20 to 30 pounds of salt per year. Some would say that salt is the only nutrient that cows have the nutritional wisdom to consume at a level to meet their requirements.

Magnesium (Mg)

Magnesium oxide is the most common form of supplemental magnesium used to prevent grass tetany. So if your cattle graze pastures where they have experienced grass tetany, magnesium oxide supplementation is necessary. Magnesium oxide is not very palatable and has been characterized as having a bitter taste. Livestock are often unwilling to consume it at recommended levels, making free-choice supplementation of magnesium oxide to grazing cattle sometimes a challenge. Feed companies have remedied some of the intake concerns by adding intake stimulants to get consumption to levels necessary to combat grass tetany. Grass tetany mineral can be expensive, so getting intake to the recommended levels is important.

Grass tetany occurs in cattle most frequently in the early spring. Grass tetany, also termed hypomagnesemia, simply means a deficiency in Mg. Prolonged Mg deficiency results in excessive urination, erratic and nervous behavior (also called grass staggers). Grass tetany results from the consumption of lush forage, which has low levels of magnesium. The apparent depression in magnesium levels results from the high water content of rapidly growing plants. If left untreated, death can occur within several hours.

In northern regions, where producers feed more harvested forages, winter tetany can occur. Many grass hays and cereal grain hays can be low in Mg (< 0.15 percent) and high in potassium (K). When Mg levels in hay fall below 0.12 percent, cattle may become vulnerable to Mg deficiency. In addition, if calcium levels are low and potassium levels are high in these feed sources, winter tetany can result. Drought conditions often result in increased use of hays and crop residue alternatives. Drought-stressed forages typically are higher in K, which also contributes to the condition. Feeding a mineral supplement high in Mg should prevent problems associated with grass or winter tetany.

Forages prone to causing grass tetany are deficient in magnesium and sodium and have an excess of potassium. Sodium is involved in transporting magnesium into cells, so it is critical to maintain adequate sodium (sodium can be supplied from salt) to facilitate proper magnesium utilization. Excess potassium consumption interferes with magnesium absorption from the gut, thus further exacerbating the condition of low dietary magnesium. In areas where grass tetany is prevalent, it is critical to consider dietary magnesium intake but also dietary levels of sodium and potassium.

Mineral Program Considerations

Soils mineral profiles impact mineral profile in the forage and therefore impact the minerals that need supplemented. Also, forage maturity impact mineral composition. The 1996 NRC for Beef Cattle suggests there are at least 17 minerals required by beef cattle. There are minerals that interact with one-another and therefore impact their utilization and these need to be factored into a supplementation strategy.
Phosphorus is a required mineral by beef cattle and is one of the most expensive minerals in a mineral supplement. Phosphorus needs for beef cows is impacted by milk production and forage type. High milk females require more compared to average- to low-milk females.

The 1996 NRC indicates that the phosphorus needs average about 22 grams per day for the first 6 months post-calving, with a range from 25 grams per day to 18 grams per day for a female with a peak milk production of 18 lb per day and weighs 1200 lb. If she consumes 2.5% of her body weight dry matter basis during June grazing and 2.3% in August and 2.1% in October and summer range is 0.17%, 0.16%, and 0.15% phosphorus in June, August, and October, how much P is needed from the mineral supplement?

In June, the cow is getting 23 gram/day from the forage, so if she eats 2 oz. of a 6% P mineral supplement it would supply another 3.4 gram (2 oz/16 oz per lb x 454 gr/lb x .06 P in the mineral = 3.4 gr) of P which meets her P requirement.

In August assume that the average dam is 5 months post-calving and she needs 19 to 20 gr/day P. Calculations based on intake and P content of the forage indicates she is getting 20 gram/day from the forage (1200 lb cow x .023 x .0016 x 454 grams/lb = 20 gr per day).

In October, milk production has declined substantially, and warm-season range quality is also declining and the cow needs 17 to 18 grams of P per day. If she is consuming 2.1% of per body weight and the forage is .15% P, she is getting 17 gram per day from the forage.

Phosphorus supplementation for cows grazing dormant range in the Sandhills appears not to be necessary. If cows consume 2.0% of their body weight on a dry matter basis, they will consume about 17 grams of P daily. In addition, the Ca:P ratio is 1.7:1 which is in the acceptable range for beef cattle of 1.4:1 to 2.0:1.

As for cows grazing corn stalks, the need for P supplementation depends on the residue left in the field. In most grazing situations, cows will get plenty of P, but the Ca:P ratio is a little high, close to 3:1. If that bothers you, then target cows to consume 3 ounces of a 6% P mineral, to get the ratio very close to 2:1.

Dr. Rick RasbyDr. Rick Rasby, Professor of Animal Science
Animal Science, University of Nebraska - Lincoln, Lincoln, NE


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